The subcellular mechanisms which underly sodium-dependent uphill solute reabsorption in the proximal convoluted tubule will be investigated. We will employ purified brush border membrane vesicles isolated from the rabbit renal cortex to quantitate the dependence of glucose, phenylalanine and inorganic phosphate transport on the electrical and chemical components of the luminal membrane sodium gradient. The coupling between sodium flux and the flux of each of these solutes will be measured. We will evaluate the hypothesis that transport interactions among these solutes arise from common interaction with the sodium gradient. As the result of these studies, it should be possible to develop a model for the cotransport with sodium of each of the three solutes. The subcellular mechanisms of uphill transport elucidated by this project will be of general biological interest. Moreover, these mechanisms may be applicable to several other solutes also reabsorbed in the proximal tubule. Current knowledge of the factors controlling sodium reabsorption in this nephron segment will also be enhanced.